This invention relates in general to photographic lens systems. More specifically it relates to multi-element photographic lenses having a movable element to provide an adjustable focus over a wide focus scale. In particular, the lens of this invention is advantageously made of plastic with high volume production techniques and for self-developing cameras.
One of the more common expedients for broadening the capabilities of a camera, whether it is a conventional camera or a camera designed for use with self-developing film, is to provide an adjustable focus. Most conventional 35 mm cameras have a unit focus where all of the optical elements forming the lens move axially in unison to change the focus. An alternative arrangement is to change the focus through the movement of a single lens element, usually the front element. Front element focus has numerous advantages over unit focus, including a fixed back focal length and reduced travel. The fixed back focal length is particularly advantageous in rigid self-developing cameras, and where it is desired to position an aperture stop behind the rear lens element. The reduced travel can eliminate or reduce the travel of an expansible bellows system together with mechanical linkages to exposure mechanisms.
Despite these clear advantages, unit focus is often used because it is more difficult to design a lens that corrects important aberrations and maintains corrections when one of its elements is moved axially with respect to the others. In the case of a triplet lens, there are just enough degrees of freedom to achieve a desired focal length and the necessary level of correction. When one of the degrees of freedom, in this case the spacing or air gap between the front and second elements, is not fixed, the design problem is significantly more difficult. Moreover, the solution may achieve a tolerable level of correction over a focus range at the expense of other performance criteria of the lens such as its aperture ratio, or cost considerations such as an increased number of elements.
U.S. Pat. No. 3,578,847 to Grey discloses a front element focusing telephoto triplet that has proven highly successful for use in conjunction with rigid self-developing cameras. A significant advantage of this design, in addition to those mentioned above, is that the lens elements are formed of plastic materials that lend themselves to low cost, high volume production techniques. Another significant advantage is that the lens is a self-contained modular unit that can be removed and replaced separately from the exposure mechanism. While this lens has a relatively large focus scale, i.e., infinity to approximately 3.5 feet, its focus range is only about one-third of that of the front element focus lens employed in the camera sold by the Polaroid Corporation under the registered designation SX-70. The latter lens can focus to approximately 10.4 inches, but requires a costly high index glass for its front element.
For reasons of both design convenience and manufacturing cost, refracting surfaces of the lens elements are almost always spherical. Aspheric surfaces, however, are well known in optical systems for correcting spherical aberration. Aspheric mirrors, for example, are used routinely to correct spherical aberration in reflecting telescopes. Aspheres have also been employed on photographic lens elements. However, since it is straightforward to correct low order spherical aberration with additional lens elements, particularly in zoom lenses where there are necessarily a large number of elements, it is rarely necessary to incur the substantial cost of grinding and polishing aspheres on glass. Thus, traditionally aspheres are employed in lens design only where higher order aberrations become significant, usually when a very fast lens is desired, or when the aspheres can justify the attendant cost by reducing the number of lens elements. For example, U.S. Pat. Nos. 2,552,672 to Grey, 3,762,801 to Baker and 3,910,685 to DeJager all describe aspheric surfaces on glass lens elements to control higher order spherical aberration. It is significant to note that these aspheres are relatively weak, that is, the departure from sphericity is not severe.
In recent years there has been an increasing use of lens elements formed of plastic materials that have significant advantages compared to glass. In plastic lenses weak aspheres have been used in their well-known roles to correct higher order aberrations in fast lenses or to reduce the number of elements in the lens. For example, U.S. Pat. No. 3,905,686 to Ruben discloses aspheres on a negative middle element of a plastic projection lens to control higher order spherical aberration. This lens has a large aperture ratio, f/1.8. Similarly, U.S. Pat. No. 3,912,379 to DeJager discloses a triplet having a plastic negative element with two aspheric surfaces that control spherical aberration at high relative apertures. U.S. Pat. No. 3,902,792 to Plummer, one of the present applicants, discloses an asphere on a fixed focus landscape lens having only one element.
Along the same line, U.S. Pat. No. 3,972,592, also issued to Ruben, discloses a six element zoom lens having an asphere formed on a second plastic element that translates to affect a change in magnification of the lens. As noted above, with six elements, lower order spherical aberration is readily corrected without aspheres. Accordingly, the Ruben zoom lens utilizes a relatively weak aspheric surface to correct higher order aberrations while providing a relatively high aperture ratio, f/1.57, with relatively few elements. As the second element moves over its range of magnification, the entire lens is unit focused with a corresponding change in the back focal length. Focus adjustment is likewise effected by unit focus in the Ruben U.S. Pat. No. 3,905,686 lens and in the DeJager U.S. Pat. No. 3,910,685 and U.S. Pat. No. 3,912,379 lenses. In these lenses, the element or elements having the aspheric surface or surfaces are fixed with respect to the other lens elements.
It is therefore a principal object of this invention to provide a multi-element photographic lens with single element focus that has an improved focus range.
Another principal object of this invention is to provide such a lens where the lens elements are formed of plastic materials.
Yet another object of this invention is to provide a lens with these advantages that has a fixed back focal length and is suitable for use in a rigid self-developing camera.
A further object is to provide such a lens that also has a relatively low cost of manufacture and lends itself to high volume production techniques.
Yet another object is to provide such a lens with an aperture ratio of approximately f/9.2 for good depth of field and which can focus from infinity to approximately two feet.